1. Field of the Invention
This invention relates to an optical fiber cable having a stack of a plurality of ribbon coated optical fibers.
2. Description of the Related Art
A so-called SZ type optical fiber cable is known for its structure allowing the coated optical fibers to be taken out with ease from the middle of the cable. The optical fiber cable of this type comprises a cylindrical center spacer having on its surface S- or Z-shaped helical grooves reversing their direction at a given pitch in which coated optical fibers are buried as disclosed in JP-A-63-301911 (the term "JP-A" as used herein means an unexamined Japanese patent publication). After coated optical fiber are put in the grooves, a filamentous material, such as nylon fiber, is helically wound onto the spacer at a certain pitch. The filament-wound spacer can further be wound with tape of, e.g., nonwoven fabric or covered with an aluminum layer. The winding of the filament, tape, and aluminum layer will hereinafter be inclusively referred to as a winding. A sheath is then provided around the wound spacer by extrusion coating.
A ribbon stack should be put in the groove in such a manner as to minimize strain energy due to bending or twisting. The strain energy is least when the ribbons are little twisted. JP-A-8-211263 proposes to make both width and depth of the groove greater than the length of the diagonal of a ribbon stack so that the ribbon stack may be placed therein smoothly.
In practice, however, because the ribbon stack is forced to bend along the SZ-shaped track, the inner surface of the groove restricts the ribbon stack. In other words, a mere increase in size of the groove does not allow the ribbon stack to move freely in the groove. In fact the lowermost ribbon optical fiber (of the ribbon stack) is in contact with the bottom of the groove at a helical portion between two adjacent reverse portions where the helical groove reverses its direction (hereinafter simply referred to as a helical portion), whereas the uppermost ribbon optical fiber comes into contact with the side wall of the groove on the inner side of the curvature at a reverse portion where the groove reverses its direction. The angle of twist of the ribbon optical fibers in the groove is decided by the position of the helical portion and that of the reverse portions.
A conventional technique is shown in FIGS. 11A to 11C. FIG. 11A is a perspective view of a spacer having S-shaped grooves. FIG. 11B is an enlarged cross section of one of the grooves at a helical portion. FIG. 11C is an enlarged cross section of the groove at a reverse portion. In the drawings, reference numeral 11 designates a spacer; 12, a high-tensile member; 13, an S-shaped grooves; and 14, a ribbon coated optical fiber. For the sake of better understanding, the lowermost ribbon optical fiber is blacked out, and the uppermost ribbon optical fiber is marked with crossing lines. While not shown in FIG. 11A, the spacer 11 has a plurality of grooves 13. Symbols S.sub.1 and S.sub.2 indicate reverse portions, and symbol S.sub.0 a helical portion.
As shown in FIG. 11B, the lowermost ribbon optical fiber is in contact with the bottom of the groove at the helical portion of the helical portion between adjacent reverse portions. At the reverse portion, the uppermost ribbon optical fiber comes into contact with the side wall of the groove on the inner side of the curvature as shown in FIG. 11C. As the groove has a rectangular section, the side walls at the reverse portion are in parallel with the line connecting the center of the groove and the center of the spacer. No consideration is given to the angle of the side walls. Because of impropriety of this angle, the ribbon stack is twisted at a considerable angle per unit length. It follow that a heavy strain is imposed on the ribbon stack, which results in an increase of transmission loss.